Lubricants such as ATF, CVTF, and MTF for transmissions of vehicles exhibit lower viscosity at higher temperatures. This means that the viscosity of these lubricants is highly temperature-dependent.
In practical applications, these lubricants are used in a wide temperature range from lower to higher temperatures. In the temperature range for use of these lubricants, changes in the viscosity of these lubricants are desirably minimized. In other words, higher viscosity indexes are preferred. As a means to improve fuel economy, a lubricant with lower viscosity has been recently used to reduce viscosity resistance. However, a lubricant with lower viscosity causes various problems such as oil leakage and seizure.
Meanwhile, use of a viscosity index improver has been proposed as another means to improve fuel economy. A lubricant having a higher viscosity index has lower viscosity resistance, which leads to improved fuel economy. Thus, it is a common practice to add a viscosity index improver to a lubricant to modify the temperature dependence of the viscosity. Known examples of such viscosity index improvers include methacrylate copolymers (Patent Literatures 1 to 4), an olefin copolymer (Patent Literature 5), and a macromonomer copolymer (Patent Literature 6).
Yet, the viscosity index improving effect of these lubricant compositions is still insufficient. In actual applications, these lubricant compositions exhibit insufficient shear stability after long hour drive and insufficient low-temperature characteristics at start-up.